1. The Field of the Invention
The present invention relates to methods of improving transmissions in optical fibers and, more particularly, to methods and apparatuses for decreasing pulse degradation and bit sequence deterioration resulting from random chromatic dispersion in optical fibers.
2. The Relevant Technology
Optical fiber communication systems, especially high-bit rate communication systems, are under active development worldwide. Numerous techniques and apparatuses are being reported for achieving high-bit rates for local area, metropolitan, and long haul optical communications. However, in transmitting information through the optical fibers at high-bit rates the optical signal degrades due to effects of chromatic dispersion (phase change of the optical signal induced by chromatic dispersion) and fiber Kerr nonlinearity (self phase modulation induced by Kerr nonlinearity).
Stable optical solitons result from exact compensation of self phase modulation and phase change due to chromatic dispersion. Soliton pulse propagation has been proposed as a method for transmitting of the information bits in optical fiber telecommunication systems in the presence of chromatic dispersion and Kerr nonlinearity. For example, U.S. Pat. No. 5,558,921, which is hereby incorporated by this reference, discloses a soliton-based single frequency optical fiber communication system with uniformly distributed fiber chromatic dispersion. Further enhancement of soliton-based optical fiber systems by implementing of multi-frequency channel technique (wavelength division multiplexingxe2x80x94WDM), however, is limited due to severe nonlinear inter-channel interaction.
U.S. Pat. No. 6,011,638, which is hereby incorporated by this reference, discloses a method that can effectively eliminate inter-channel interaction in soliton based telecommunication systems with lumped in-line optical amplifiers through proper dispersion management. In one embodiment of this method, the fiber chromatic dispersion decreases exponentially along a fiber as the energy of optical signal decreases. In another embodiment of this method, the exponentially decreasing profile is approximated in a step-wise manner using fiber spans with different values of chromatic dispersion uniformly distributed over each span.
A method for significantly improving the performance of optical communication system was proposed in the paper xe2x80x9cOptical-pulse equalization of low-dispersion transmission in single-mode fibers in the 1.3-1.7-xcexcm spectral regionxe2x80x9d by C. Lin, H. Kogelnik and L. G. Cohen, Opt. Lett. 5, 476 (1980). This method reduces signal deterioration due to chromatic dispersion by means of periodical inserting additional fiber spans with the opposite sign of the dispersion (dispersion management) that are required to keep the overall dispersion within one period close to zero. U.S. Pat. No. 5,471,333, which is hereby incorporated by this reference, discloses a method that eliminates pulse deterioration due to Kerr self-phase modulation by using the effects of the chromatic dispersion within one period of the dispersion map and provides stable oscillating pulses (dispersion managed solitons) was disclosed by. Dispersion managed solitons are stable, compatible with the WDM technique, and well suited for high bit rate telecommunications. Consequently, dispersion managed solitons have been proposed as a method for transmitting the information bits in optical fiber telecommunication systems with dispersion management. For example, U.S. Pat. No. 6,243,181, which is incorporated by this reference, discloses a method of reducing the inter-channel interaction in the soliton based optical communication systems with dispersion management.
The value of chromatic dispersion in the fiber spans of soliton based telecommunication systems is considered to be deterministic (predictable). However, the dispersion is known to randomly vary along the fiber, for example, in dispersion shifted fibers. Random variation of the zero dispersion wavelength was indicated in xe2x80x9cFour-wave mixing in an optical fiber in the zero-dispersion wavelength regionxe2x80x9d by K. Inoue, J. Lightwave Technol. 10, pp. 1553-1561 (1992). Variation of the zero dispersion point was obtained by cutting a 10 km length of dispersion shifted fiber into four 2.5 km segments and subsequent measurement of the average zero dispersion wavelength. The randomness of fiber chromatic dispersion was demonstrated using a nondestructive accurate method of dispersion measurement along a fiber, which method has been disclosed in U.S. Pat. No. 5,956,131, which is hereby incorporated by this reference.
Historically unrecognized effects of random chromatic dispersion have the potential for uncontrolled growth of additional pulse deterioration due to the presence of randomness in fiber chromatic dispersion. Such uncontrolled growth of the pulse deterioration imposes penalties in the transmission system by two different mechanisms. The first mechanism is optical pulse broadening that eventually deteriorates information bit pattern. The second mechanism is shedding of nonlocal continuous radiation from localized optical pulses. Since this continuous radiation is nonlocal, optical pulses experience interaction through continuous radiation, resulting in increased separation between pulses. These penalties rapidly increase with shortening of the pulse width, i.e., when the bit-rate is increased.
Refining fiber optic production technology is expensive and currently does not offer an absolute cure to the adverse effects of random chromatic dispersion. Given the ever-increasing demand for higher transmission rates, minimizing pulse degradation in optical fibers resulting from random chromatic dispersion is important to increasing the bandwidth of optical fiber.
Accordingly, a need exists for methods and optical fibers for decreasing pulse degradation resulting from random chromatic dispersion in optical fibers. It would be a further advancement in the state of the art fiber optics technology to provide such a method and apparatus in a cost-effective manner. It would also be an advancement in the art to provide a method of minimizing pulse degradation resulting from random chromatic dispersion in the existing and newly manufactured cables.
The present invention provides methods and apparatuses for decreasing pulse degradation resulting from random chromatic dispersion in optical fibers. More specifically, the present invention provides methods for periodically pinning (approximating) an actual (random) accumulated chromatic dispersion to a predicted (nominal) accumulated dispersion through relatively simple modifications of fiber-optic manufacturing methods or retrofitting of existing fibers. Through use of these methods and apparatuses, increased optical transmission speeds may be enabled.
If the pinning occurs with sufficient frequency (at a distance less than or equal to a correlation scale, Z"xgr"), pulse degradation resulting from random chromatic dispersion is minimized. The correlation scale may be defined by the following equation:
Z"xgr"=xcfx844/D,
where xcfx84 is a pulse (bit of information) width at which signal is launched into an optical fiber and D indicates dispersion noise strength. Dispersion noise strength is a measure of the variation of actual (random) dispersion relative to predicted (nominal) dispersion.
Pinning may occur periodically (less than or equal to the correlation scale) or quasi-periodically along the length of the optical fiber. Quasi-periodic pinning may involve pinning at irregular intervals. For example, with quasi-periodic pinning, a distance between each consecutive instance of pinning may be between approximately zero and approximately two to three times the correlation scale. In another embodiment, such a distance may be between one half of the correlation scale and one and one half times the correlation scale.
Pinning involves points (xe2x80x9cpinning pointsxe2x80x9d) along an optical fiber where actual accumulated dispersion approximates (is equal to or nearly equal to) predicted accumulated dispersion. Pinning may involve naturally occurring pinning points or pinning points resulting from the use of compensating strands of optical fiber. A compensating strand of optical fiber may be used to alter actual accumulated dispersion such that it approximates predicted accumulated dispersion.
In one embodiment, which implements naturally occurring pinning points, a predicted accumulated dispersion may be determined from a first point along a first optical fiber. Then, the actual accumulated dispersion from the first point along the first optical fiber may be determined.
Thereafter, a pinning point, where the actual accumulated dispersion approximates the predicted accumulated dispersion, may be located. Next, an optical fiber segment spanning from the first point to the pinning point may be formed.
In one implementation, a plurality of optical fiber segments may be formed, as described above. The plurality of optical fiber segments may be joined to form a second optical fiber using any suitable method.
As explained above, a length of each optical fiber segment may approximate or be less than the correlation scale. Alternatively, the length of each optical fiber segment may be distributed between approximately zero and approximately two to three times the correlation scale (quasi-periodic).
The second optical fiber minimizes pulse degradation resulting from random chromatic dispersion. In fact, if the pinning period is short enough, a statistically stable pulse may be transmitted.
In an alternative embodiment of this invention, a first plurality of optical fiber segments may be formed (extracted) from a first optical fiber. A length of each of the first plurality of optical fiber segments may be less than or approximate the correlation scale. Alternatively, the length of each of the first plurality of optical fiber segments may be distributed between approximately zero and approximately two to three times the correlation scale (quasi-periodic pinning).
Then, the predicted accumulated chromatic dispersion along each of the first plurality of optical fiber segments is determined. An actual accumulated chromatic dispersion along each of the first plurality of optical fiber segments is also determined. A pinning point is located on each of the first plurality of optical fiber segments. If no such point exists within an optical fiber segment, the segment should be discarded.
Each of the first plurality of optical fiber segments is then severed at the point to form a second plurality of optical fiber segments. Next, each of the second plurality of optical fiber segments is joined to form a second optical fiber.
Another embodiment uses compensating strands to generate pinning points. Initially, a plurality of points may be identified along an optical fiber. Thereafter, a predicted accumulated chromatic dispersion may be determined at each of the plurality of points on the optical fiber. Next, an actual accumulated chromatic dispersion may be determined at each of the plurality of points.
Thereafter, a compensating strand of optical fiber may be inserted at each of the plurality of points. Each compensating strand of optical fiber may be configured to alter the actual accumulated chromatic dispersion at an associated one of the plurality of points such that the actual accumulated chromatic dispersion approximates the predicted accumulated chromatic dispersion at an end of each compensating strand.
A distance between an end of each consecutive compensating strand may approximate or be less than a correlation scale. Alternatively, as one would expect, a distance between the end of each consecutive compensating strand may be distributed between approximately zero and approximately two to three times the correlation scale.
It should be noted that this invention, in its various embodiments, may be implemented in connection with single-mode and multi-mode fibers. It may also be implemented in connection with various types of telecommunication fibers including, but not limited to, dispersion management fiber, fiber having constant positive predicted dispersion, dispersion shifted fiber, dispersion flattened fiber, large effective area optical fiber (LEAF), optical fiber with dispersion management of second and third order dispersion, and any combination thereof. Additionally, both non-linear (soliton-based) and linear transmission methods may be used.
The features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.